To provide solar cell modules with enhanced conversion efficiency and long-term reliability over 20 to 30 years or even longer, a number of reports and proposals relating to encapsulants were made in the art. From the standpoint of efficiency enhancement, the silicone material is reported to be superior in internal quantum efficiency due to light transmittance at wavelength of about 300 to 400 nm, as compared with the ethylene-vinyl acetate copolymer (EVA) which is currently the mainstream of encapsulant (see Non-Patent Document 1, for example). In fact, an experiment to compare the output power of solar modules using EVA and silicone material as encapsulant is reported (see Non-Patent Document 2, for example). From the standpoint of long-term reliability, it is reported that modules using silicone as the encapsulant experience only a percent deterioration of maximum power as low as −0.22%/year even after 29-year outdoor exposure (see Non-Patent Document 3, for example).
Originally, the use of silicone material as encapsulant was already implemented in the early period of 1970s when solar cell modules for spacecraft were fabricated. Historically, in the stage when solar cell modules for ground applications were manufactured, the silicone material was replaced by EVA because the silicone material had outstanding problems including material cost and workability for encapsulation whereas the EVA was inexpensive and supplied in film form. Recently, the efficiency enhancement and long-term reliability of solar cells are highlighted again. Accordingly, the properties of silicone material as encapsulant, for example, low modulus, high transparency and weather resistance are considered valuable again. Several encapsulating methods using silicone material are newly proposed.
With respect to the use of silicone sheets, for example, Patent Document 1 discloses encapsulation using a sheet of organopolysiloxane-based hot melt material. However, it is difficult to work the polysiloxane into a sheet while maintaining high transparency. When the polysiloxane is shaped into a sheet of about 1 mm thick, for example, only a particular shaping technique such as casting or pressing is applicable due to the “brittleness” of the material. This shaping technique is unsuitable for mass-scale production. Patent Document 2 proposes a thermoplastic silicone sheet made of a polysiloxane-urea base copolymer. This copolymer may be inferior to the polysiloxane with respect to transparency on the short-wavelength side, and a more cost be required for copolymer preparation.
With respect to the use of liquid silicone material, Patent Document 3 discloses that interconnected solar cells are positioned on or in a liquid silicone material coated on a substrate, using a multi-axis robot. The liquid silicone material is then cured, thereby achieving encapsulation without trapping air bubbles. Further, Patent Document 4 proposes that a solar cell is placed in vacuum, and the components are compressed using a cell press having a movable plate, thereby achieving encapsulation without trapping air bubbles. Since either of these methods differs significantly from the conventional solar cell encapsulating methods, there is a possibility that the existing mass-production systems cannot be used.
Another known encapsulation method is by coating two glass plates with a silicone composition, sandwiching a solar cell matrix between the coated glass plates in vacuum, and heating the assembly for curing. However, the procedure of coating the silicone composition, overlapping the coatings, and curing has several problems. Because of the low viscosity of the silicone composition, if the coated surface is faced vertically downward, the coating will flow, resulting in a variation of coating thickness. Thus, coating and curing treatments must be conducted on a horizontal platen, and the equipment used in mass-scale manufacture becomes of large size. When the coating is faced downward after curing, the coating is inhibited from flowing. However, once silicone composition coatings are cured, they are not bonded together even when overlapped. Eventually, it is required to dispense the silicone composition in vacuum and to introduce a large-size equipment.
Because of process complexity as mentioned above, the attempt to apply a silicone material to the mass-scale manufacture of solar cell modules to take advantage of the low modulus of silicone material has marked no further advances.